I was returning from New Orleans where I was attending the American Geophysical Union Fall Meeting. There was a short layover in Denver and I had a short time to find some food, which is challenging with my dietary restrictions. I cannot recall precisely, but I got some notification from my CGS crew about a magnitude M 6.2 earthquake offshore of the Mendocino triple junction. One of these notifications was from Cindy as we both collaborate to prepare quick reports for earthquakes in California. These reports are sent upstream to management in our organization and others. I was unavailable to contribute this time.
Needless to say, I was sad to have missed experiencing this good sized shaker for myself. This is the first earthquake of this size that I have missed (in Humboldt) since I moved here in 1991.
I got home about 3 am the next morning and did not have energy to prepare an earthjay report. Though I started working on it the next day. However, I soon learned that this was a complicated earthquake and I decided to await additional analyses by the Berkeley Seismmo Lab and the USGS.
Last week or so, their analyses were produced publicly and the earthquake catalog was updated. What we discovered is that there were two closely spaced (in time but not space) earthquakes, an M 5.7 and and M 6.2.
It was complicated for the seismologists to work out because the seismic waves of the two events overlapped in time. i.e., the waves from the first quake were still passing through the Earth when the waves from the second quake started.
Basically, there was initially an M 5.7 strike-slip earthquake along the Mendocino transform fault zone about 20 km (12.5 miles) offshore. About 10 or 11 seconds later, there was an M 6.2 strike-slip earthquake within the Gorda plate, below the megathrust fault.
Here is a plot from the USGS. Each horizontal squiggly line is the seismograph record from an individual seismometer. They are plotted with the seismometer closest to the earthquake on the bottom row and the furthest seismometer on the uppermost row.
The P wave (primary wave) is the first of four major types of seismic waves. Next comes the S (secondary) wave, then the Love waves, and finally the Raleigh waves.
The P wave arrives at closer seismometers before it arrives at more distant seismometers. Because of this, we generally call this type of plot a travel time plot.
In the above plot we can see how the M 6.1 P waves are arriving while the M 5.7 S waves are still being transmitted.
The M 5.7 is clearly a right-lateral strike-slip event given the aftershock pattern and the known location and type of the Mendocino fault system (a right-lateral strike-slip fault.
The interpretation for the type of earthquake for the M 6.2 is a little more complicated.
Earthquake mechanisms (the “beach balls”) show two possible ways that the earthquake could have slipped. We use aftershock patterns and existing mapped faults to help us interpret which of these [nodal] fault planes is the more likely one.
If we look at the earthquake poster below, we see that the M 6.2 earthquake is an almost pure strike-slip earthquake. The two possible fault planes are one that is oriented in the northwest direction (would be right-lateral) and one that is in the northeast direction (would be left-lateral).
There are two reasons why I interpret the M 6.2 to be right-lateral (of course, I could be wrong).
- There are a series of aftershocks that appear to align along a northwest trajectory. See the 6 mechanisms for the earthquakes just north of the M 6.2. The epicenters for these M 3.8, 3.0, 3.2, 3.0, 3.3, and 3.8 earthquakes align with a northwest oriented trend.
- The 1992 Cape Mendocino Earthquake main events began with an M 7.2 thrust type earthquake mainshock followed by two triggered events with magnitudes of M 6.6 and M 6.5. These triggered events are strike-slip and within the Gorda plate (“intraplate” events). If we take a look at the earthquake catalog after it has been modeled using a “double differencing” technique (read more here), we would notice several northwest trends in seismicity. These trends appeared after the 1992 sequence.
So, while most of our experience with the Gorda plate is with northeast oriented (striking) left-lateral strike-slip faults (e.g., 1980, 2010, 2014, etc.) it is possible that there are other faults, sub-parallel to the post-1992 seismicity trends, where the M 6.2 and other aftershocks were hosted. I mention these northwest trending faults in a recent Earthquake Report here.
Something that is interesting is that the onshore events from this 20 Dec 2021 sequence are just to the north of the aftershocks from the 1992 sequence. They are at similar depths as those ’92 quakes and have similar earthquake mechanisms. As Spock would say, Fascinating.
Dr. Anthony Lomax, famous for his work locating the hypocenter for the 1906 San Francisco Earthquake, has been developing excellent tools for seismologists ever since. He recently applied one of his new tools to locate earthquakes to the Mendocino triple junction region. I present some of his figures below.
Below is my interpretive poster for this earthquake
- I plot the seismicity from the past month, with diameter representing magnitude (see legend). I include earthquake epicenters from 1920-2020 with magnitudes M ≥ 3.0 in one version.
- I plot the USGS fault plane solutions (moment tensors in blue and focal mechanisms in orange), possibly in addition to some relevant historic earthquakes.
- A review of the basic base map variations and data that I use for the interpretive posters can be found on the Earthquake Reports page. I have improved these posters over time and some of this background information applies to the older posters.
- Some basic fundamentals of earthquake geology and plate tectonics can be found on the Earthquake Plate Tectonic Fundamentals page.
- In the upper left corner is a small scale view of the Mendocino triple junction where the southern Cascadia subduction zone meets the right-lateral Mendocino and San Andreas strike-slip faults.
- In the lower center is a plot of earthquake intensity (vertical axis) relative to distance from the earthquake (horizontal axis). The blue and orange dots represent USGS Did You Feel It? reports, observations from real people. The green and orange lines show the plots from the USGS [empirical] models of shaking intensity.
- To the left of the plot is a map that shows these same data. The colored areas are the average intensity reported by people (dyfi numbers). The colored lines represent the USGS modeled intensities. Both the plot and the map show that the shaking intensity gets smaller with distance from the earthquake.
- In the left upper center is a map that shows liquefaction susceptibility. This is a model from the USGS that uses ground shaking data to estimate where there may be liquefaction. I drove around after the earthquake and could not locate any evidence for liquefaction but my search was far from comprehensive.
I include some inset figures. Some of the same figures are located in different places on the larger scale map below.
- Here are two relevant interpretive posters from the 1992 Cape Mendocino Earthquake.
- This one is an overview of the earthquake.
- Here is a poster that shows a comparison between the 1991 Honeydew and 1992 Cape Mendocino earthquakes..
Other Report Pages
- Below are a series of maps that show the shaking intensity and potential for landslides and liquefaction. These are all USGS data products.
- Below is the liquefaction susceptibility and landslide probability map (Jessee et al., 2017; Zhu et al., 2017). Please head over to that report for more information about the USGS Ground Failure products (landslides and liquefaction). Basically, earthquakes shake the ground and this ground shaking can cause landslides. We can see that there is a low probability for landslides. However, we have already seen photographic evidence for landslides and the lower limit for earthquake triggered landslides is magnitude M 5.5 (from Keefer 1984)
- I use the same color scheme that the USGS uses on their website. Note how the areas that are more likely to have experienced earthquake induced liquefaction are in the valleys. Learn more about how the USGS prepares these model results here.
Shaking Intensity and Potential for Ground Failure
There are many different ways in which a landslide can be triggered. The first order relations behind slope failure (landslides) is that the “resisting” forces that are preventing slope failure (e.g. the strength of the bedrock or soil) are overcome by the “driving” forces that are pushing this land downwards (e.g. gravity). The ratio of resisting forces to driving forces is called the Factor of Safety (FOS). We can write this ratio like this:
FOS = Resisting Force / Driving Force
When FOS > 1, the slope is stable and when FOS < 1, the slope fails and we get a landslide. The illustration below shows these relations. Note how the slope angle α can take part in this ratio (the steeper the slope, the greater impact of the mass of the slope can contribute to driving forces). The real world is more complicated than the simplified illustration below.
Landslide ground shaking can change the Factor of Safety in several ways that might increase the driving force or decrease the resisting force. Keefer (1984) studied a global data set of earthquake triggered landslides and found that larger earthquakes trigger larger and more numerous landslides across a larger area than do smaller earthquakes. Earthquakes can cause landslides because the seismic waves can cause the driving force to increase (the earthquake motions can “push” the land downwards), leading to a landslide. In addition, ground shaking can change the strength of these earth materials (a form of resisting force) with a process called liquefaction.
Sediment or soil strength is based upon the ability for sediment particles to push against each other without moving. This is a combination of friction and the forces exerted between these particles. This is loosely what we call the “angle of internal friction.” Liquefaction is a process by which pore pressure increases cause water to push out against the sediment particles so that they are no longer touching.
An analogy that some may be familiar with relates to a visit to the beach. When one is walking on the wet sand near the shoreline, the sand may hold the weight of our body generally pretty well. However, if we stop and vibrate our feet back and forth, this causes pore pressure to increase and we sink into the sand as the sand liquefies. Or, at least our feet sink into the sand.
Below is a diagram showing how an increase in pore pressure can push against the sediment particles so that they are not touching any more. This allows the particles to move around and this is why our feet sink in the sand in the analogy above. This is also what changes the strength of earth materials such that a landslide can be triggered.
Below is a diagram based upon a publication designed to educate the public about landslides and the processes that trigger them (USGS, 2004). Additional background information about landslide types can be found in Highland et al. (2008). There was a variety of landslide types that can be observed surrounding the earthquake region. So, this illustration can help people when they observing the landscape response to the earthquake whether they are using aerial imagery, photos in newspaper or website articles, or videos on social media. Will you be able to locate a landslide scarp or the toe of a landslide? This figure shows a rotational landslide, one where the land rotates along a curvilinear failure surface.
Some Relevant Discussion and Figures
- Here is a map of the Cascadia subduction zone, modified from Nelson et al. (2006). The Juan de Fuca and Gorda plates subduct norteastwardly beneath the North America plate at rates ranging from 29- to 45-mm/yr. Sites where evidence of past earthquakes (paleoseismology) are denoted by white dots. Where there is also evidence for past CSZ tsunami, there are black dots. These paleoseismology sites are labeled (e.g. Humboldt Bay). Some submarine paleoseismology core sites are also shown as grey dots. The two main spreading ridges are not labeled, but the northern one is the Juan de Fuca ridge (where oceanic crust is formed for the Juan de Fuca plate) and the southern one is the Gorda rise (where the oceanic crust is formed for the Gorda plate).
- Here is a version of the CSZ cross section alone (Plafker, 1972). This shows two parts of the earthquake cycle: the interseismic part (between earthquakes) and the coseismic part (during earthquakes). Regions that experience uplift during the interseismic period tend to experience subsidence during the coseismic period.
- This poster includes seismicity from the past ~5 decades, for temblors M > 3.0. I also include the map and cross section as explained above. On the left is a map that shows the possible shaking intensity from a future CSZ earthquake.
- More about the materials on this poster can be found on this page.
- Hemphill-Haley, E., 1995. Diatom evidence for earthquake-induced subsidence and tsunami 300 yr ago in southern coastal Washington in GSA Bulletin, v. 107, p. 367-378.
- Nelson, A.R., Shennan, I., and Long, A.J., 1996. Identifying coseismic subsidence in tidal-wetland stratigraphic sequences at the Cascadia subduction zone of western North America in Journal of Geophysical Research, v. 101, p. 6115-6135.
- Atwater, B.F. and Hemphill-Haley, E., 1997. Recurrence Intervals for Great Earthquakes of the Past 3,500 Years at Northeastern Willapa Bay, Washington in U.S. Geological Survey Professional Paper 1576, Washington D.C., 119 pp.
I have compiled some literature about the CSZ earthquake and tsunami. Here is a short list that might help us learn about what is contained within the core that I collected.
- This figure shows how a subduction zone deforms between (interseismic) and during (coseismic) earthquakes. We also can see how a subduction zone generates a tsunami. Atwater et al., 2005.
- Here is an animation produced by the folks at Cal Tech following the 2004 Sumatra-Andaman subduction zone earthquake. I have several posts about that earthquake here and here. One may learn more about this animation, as well as download this animation here.
The Gorda and Juan de Fuca plates subduct beneath the North America plate to form the Cascadia subduction zone fault system. In 1992 there was a swarm of earthquakes with the magnitude Mw 7.2 Mainshock on 4/25. Initially this earthquake was interpreted to have been on the Cascadia subduction zone (CSZ). The moment tensor shows a compressional mechanism. However the two largest aftershocks on 4/26/1992 (Mw 6.5 and Mw 6.7), had strike-slip moment tensors. In my mind, these two aftershocks aligned on what may be the eastern extension of the Mendocino fault. However, looking at their locations, my mind was incorrect. These two earthquakes were not aftershocks, but were either left-lateral or right-lateral strike-slip Gorda plate earthquakes triggered by the M 7.1 thrust event.
These two quakes appear to be aligned with the two northwest trends in seismicity and the 18 March 2020 M 5.2. The orientation of the mechanisms are not as perfectly well aligned, but there are lots of reasons for this (perhaps the faults were formed in a slightly different orientation, but have rotated slightly).
There have been several series of intra-plate earthquakes in the Gorda plate. Two main shocks that I plot of this type of earthquake are the 1980 (Mw 7.2) and 2005 (Mw 7.2) earthquakes. I place orange lines approximately where the faults are that ruptured in 1980 and 2005. These are also plotted in the Rollins and Stein (2010) figure above. The Gorda plate is being deformed due to compression between the Pacific plate to the south and the Juan de Fuca plate to the north. Due to this north-south compression, the plate is deforming internally so that normal faults that formed at the spreading center (the Gorda Rise) are reactivated as left-lateral strike-slip faults. In 2014, there was another swarm of left-lateral earthquakes in the Gorda plate. I posted some material about the Gorda plate setting on this page.
- Here is a link to the embedded video below, showing the week-long seismicity in April 1992.
- This is the map used in the animation below. Earthquake epicenters are plotted (some with USGS moment tensors) for this region from 1917-2017 with M ≥ 6.5. I labeled the plates and shaded their general location in different colors.
- I include some inset maps.
- In the upper right corner is a map of the Cascadia subduction zone (Chaytor et al., 2004; Nelson et al., 2004).
- In the upper left corner is a map from Rollins and Stein (2010). They plot epicenters and fault lines involved in earthquakes between 1976 and 2010.
- Here is a map from Rollins and Stein, showing their interpretations of different historic earthquakes in the region. This was published in response to the Januray 2010 Gorda plate earthquake. The faults are from Chaytor et al. (2004).
- Here is a large scale map of the 1994 earthquake swarm. The mainshock epicenter is a black star and epicenters are denoted as white circles.
- Here is a plot of focal mechanisms from the Dengler et al. (1995) paper in California Geology.
- In this map below, I label a number of other significant earthquakes in this Mendocino triple junction region. Another historic right-lateral earthquake on the Mendocino fault system was in 1994. There was a series of earthquakes possibly along the easternmost section of the Mendocino fault system in late January 2015, here is my post about that earthquake series.
- Here is a map from Chaytor et al. (2004) that shows some details of the faulting in the region. The moment tensor (at the moment i write this) shows a north-south striking fault with a reverse or thrust faulting mechanism. While this region of faulting is dominated by strike slip faults (and most all prior earthquake moment tensors showed strike slip earthquakes), when strike slip faults bend, they can create compression (transpression) and extension (transtension). This transpressive or transtentional deformation may produce thrust/reverse earthquakes or normal fault earthquakes, respectively. The transverse ranges north of Los Angeles are an example of uplift/transpression due to the bend in the San Andreas fault in that region.
- These are the models for tectonic deformation within the Gorda plate as presented by Jason Chaytor in 2004.
Tectonic configuration of the Gorda deformation zone and locations and source models for 1976–2010 M ≥ 5.9 earthquakes. Letters designate chronological order of earthquakes (Table 1 and Appendix A). Plate motion vectors relative to the Pacific Plate (gray arrows in main diagram) are from Wilson , with Cande and Kent’s  timescale correction.
A: Mapped faults and fault-related ridges within Gorda plate based on basement structure and surface morphology, overlain on bathymetric contours (gray lines—250 m interval). Approximate boundaries of three structural segments are also shown. Black arrows indicated approximate location of possible northwest- trending large-scale folds. B, C: uninterpreted and interpreted enlargements of center of plate showing location of interpreted second-generation strike-slip faults and features that they appear to offset. OSC—overlapping spreading center.
Models of brittle deformation for Gorda plate overlain on magnetic anomalies modified from Raff and Mason (1961). Models A–F were proposed prior to collection and analysis of full-plate multibeam data. Deformation model of Gulick et al. (2001) is included in model A. Model G represents modification of Stoddard’s (1987) flexural-slip model proposed in this paper.
- 1700.09.26 M 9.0 Cascadia’s 315th Anniversary 2015.01.26
- 1700.09.26 M 9.0 Cascadia’s 316th Anniversary 2016.01.26 updated in 2017 and 2018
- 1992.04.25 M 7.1 Cape Mendocino 25 year remembrance
- 1992.04.25 M 7.1 Cape Mendocino 25 Year Remembrance Event Page
- Earthquake Information about the CSZ 2015.10.08
- 2020.05.18 M 5.5 Gorda Rise
- 2017.07.28 M 5.1 Gorda plate
- 2010.01.10 M 6.5 Gorda plate
- 2019.08.29 M 6.3 Blanco transform fault
- 2018.08.22 M 6.2 Blanco transform fault
- 2018.07.29 M 5.3 Blanco transform fault
- 2015.06.01 M 5.8 Blanco transform fault p-1
- 2015.06.01 M 5.8 Blanco transform fault p-2 (animations)
- 2021.12.20 M 5.7 & 6.2 Mendocino fault
- 2020.03.09 M 5.8 Mendocino fault
- 2018.01.25 M 5.8 Mendocino fault
- 2017.09.22 M 5.7 Mendocino fault
- 2016.12.08 M 6.5 Mendocino fault, CA
- 2016.12.08 M 6.5 Mendocino fault, CA Update #1
- 2016.12.05 M 4.3 Petrolia CA
- 2016.10.27 M 4.1 Mendocino fault
- 2016.09.03 M 5.6 Mendocino
- 2016.01.02 M 4.5 Mendocino fault
- 2017.03.06 M 4.0 Cape Mendocino
- 2015.10.29 M 3.4 Bayside
- 2017.06.11 M 3.5 Gorda or NAP?
Cascadia subduction zone
Blanco transform fault
Mendocino triple junction
North America plate
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- Jessee, M.A.N., Hamburger, M. W., Allstadt, K., Wald, D. J., Robeson, S. M., Tanyas, H., et al. (2018). A global empirical model for near-real-time assessment of seismically induced landslides. Journal of Geophysical Research: Earth Surface, 123, 1835–1859. https://doi.org/10.1029/2017JF004494
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- Müller, R.D., Sdrolias, M., Gaina, C. and Roest, W.R., 2008, Age spreading rates and spreading asymmetry of the world’s ocean crust in Geochemistry, Geophysics, Geosystems, 9, Q04006, https://doi.org/10.1029/2007GC001743
- Pagani,M. , J. Garcia-Pelaez, R. Gee, K. Johnson, V. Poggi, R. Styron, G. Weatherill, M. Simionato, D. Viganò, L. Danciu, D. Monelli (2018). Global Earthquake Model (GEM) Seismic Hazard Map (version 2018.1 – December 2018), DOI: 10.13117/GEM-GLOBAL-SEISMIC-HAZARD-MAP-2018.1
- Silva, V ., D Amo-Oduro, A Calderon, J Dabbeek, V Despotaki, L Martins, A Rao, M Simionato, D Viganò, C Yepes, A Acevedo, N Horspool, H Crowley, K Jaiswal, M Journeay, M Pittore, 2018. Global Earthquake Model (GEM) Seismic Risk Map (version 2018.1). https://doi.org/10.13117/GEM-GLOBAL-SEISMIC-RISK-MAP-2018.1
- Zhu, J., Baise, L. G., Thompson, E. M., 2017, An Updated Geospatial Liquefaction Model for Global Application, Bulletin of the Seismological Society of America, 107, p 1365-1385, https://doi.org/0.1785/0120160198
- Atwater, B.F., Musumi-Rokkaku, S., Satake, K., Tsuju, Y., Eueda, K., and Yamaguchi, D.K., 2005. The Orphan Tsunami of 1700—Japanese Clues to a Parent Earthquake in North America, USGS Professional Paper 1707, USGS, Reston, VA, 144 pp.
- Chaytor, J.D., Goldfinger, C., Dziak, R.P., and Fox, C.G., 2004. Active deformation of the Gorda plate: Constraining deformation models with new geophysical data: Geology v. 32, p. 353-356.
- Dengler, L.A., Moley, K.M., McPherson, R.C., Pasyanos, M., Dewey, J.W., and Murray, M., 1995. The September 1, 1994 Mendocino Fault Earthquake, California Geology, Marc/April 1995, p. 43-53.
- Geist, E.L. and Andrews D.J., 2000. Slip rates on San Francisco Bay area faults from anelastic deformation of the continental lithosphere, Journal of Geophysical Research, v. 105, no. B11, p. 25,543-25,552.
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- McCrory, P.A.,. Blair, J.L., Waldhauser, F., kand Oppenheimer, D.H., 2012. Juan de Fuca slab geometry and its relation to Wadati-Benioff zone seismicity in JGR, v. 117, B09306, doi:10.1029/2012JB009407.
- McLaughlin, R.J., Sarna-Wojcicki, A.M., Wagner, D.L., Fleck, R.J., Langenheim, V.E., Jachens, R.C., Clahan, K., and Allen, J.R., 2012. Evolution of the Rodgers Creek–Maacama right-lateral fault system and associated basins east of the northward-migrating Mendocino Triple Junction, northern California in Geosphere, v. 8, no. 2., p. 342-373.
- Nelson, A.R., Asquith, A.C., and Grant, W.C., 2004. Great Earthquakes and Tsunamis of the Past 2000 Years at the Salmon River Estuary, Central Oregon Coast, USA: Bulletin of the Seismological Society of America, Vol. 94, No. 4, pp. 1276–1292
- Rollins, J.C. and Stein, R.S., 2010. Coulomb stress interactions among M ≥ 5.9 earthquakes in the Gorda deformation zone and on the Mendocino Fault Zone, Cascadia subduction zone, and northern San Andreas Fault: Journal of Geophysical Research, v. 115, B12306, doi:10.1029/2009JB007117, 2010.
- Stoffer, P.W., 2006, Where’s the San Andreas Fault? A guidebook to tracing the fault on public lands in the San Francisco Bay region: U.S. Geological Survey General Interest Publication 16, 123 p., online at http://pubs.usgs.gov/gip/2006/16/
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Basic & General References
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